Hungarian police have revealed that during their investigations into the deaths of 71 refugees found in a refrigerated truck, a “vast and ruthless network" of human traffickers profiting from the migrant crisis had been uncovered.

In August, 2015, a Volvo truck driven from Kecskemét, a town in central Hungary on the main highway to the Serbian border, picked up the migrants. The gang charged between 1,200 and 1,500 euros per person for smuggling them from Serbia to Germany, according to police.

With a passenger car ahead and another behind the truck, the convoy set out on the journey, rigorously observing traffic rules.

By the time they arrived near Budapest, the migrants had suffocated. The drivers eventually abandoned the truck by the side of a highway in Austria.

Zoltán Boross, the head of Hungary’s migration police department, said he would recommend pressing murder and smuggling charges against one Afghan and seven Bulgarians suspected of arranging the transport of the men, women and chidren, who had suffocated to death.

“The unscrupulousness and greed of the Afghan leader of this criminal gang is what led to this tragedy," Boross said, in an extract taken from the Wall Street Journal.

When the inflow of migrants, mostly from the Middle East and Afghanistan, started rising last year, the suspects began buying bigger trucks, Boross said. In total, they are suspected of having ferried over 1,100 people across Hungary.

Authorities haven’t yet disclosed the identities of the suspects, who were detained in Hungary last year.

Hungarian and European Union police said that although the human flow of migrants into Europe has dropped significantly compared with last year, people trafficking remains big business.

Europol, the EU police agency, said migrant trafficking generated estimated revenue of more than 5 billion euros in 2015. More than 12,000 new migrant-smuggling suspects have been identified since the start of the year.

“You see a number of multinational criminal gangs operating across Europe, making their profit regardless of the risks put to migrants’ lives," said Robert Crepinko, head of Europol’s anti-smuggling division.